With steady technological progress, demands on precision of molding are increasing by the day. Of particular importance are speed and aiming precision of injection molding machines. Previously, injection pressure was generated by hydraulic devices or servomotors. Mechanic transmission, however, effects hysteresis and backlash, so that injection is not aimed precisely and demands of high technology are not met. During metering, the driving unit of the injection screw moves along with the injection driving device, greatly increasing inertia. In addition, the accuracies at the end of movement is reduced, thereby reducing the precision in molding. Thus, the present invention developed to eliminate the positional inaccuracies and injectional backlash generated by mechanic transmission and high inertia, thereby achieving high speed and precision molding.
1. Basic Object of the Present Invention
An electromagnetic coaxial driving injection apparatus is provided, comprising an injection screw; an injection and metering driving device coaxial therewith, at least one connecting unit, and at least one bearing. When the injection screw performs a linear movement, only rotors of motors move along, while stators of the motors remain at rest, greatly reducing inertia. Both injection and metering are directly driven, without any transmission by belt or gears, so that no backlash and hysteresis occur. Since the number of structural parts are lessened, accumulating inaccuracies due to manufacturing and assembly are avoided. The present invention thus meets demands for high speed and precision.
2. Description of Related Art
A conventional injection system of an injection molding machine comprises an injection driving device for driving a linear movement of the injection system and a metering device for driving and controlling a rotating movement of an injection screw of the injection system to perform metering.
Linear driving for injection systems of injection molding machines is performed either hydraulically or electrically. Hydraulic devices allow readily to implement a linear driving force easily. However, due to the compressibility behavior of fluid and change of properties thereof with temperature, positioning accuracy is comparatively poor. Furthermore, noise is generated during operation, cleaning is difficult and power consumption is high. An electric motor generating a rotational motion, on the other hand, consumes less power than a hydraulic device and is accurately controllable, but transformation of a rotational movement into a linear movement is needed. Therefore, a complicated mechanism of belt and pulleys is required. This causes the structure to be complicated and requirement for space to be high.
Using a linear motor as a prime driving force for driving injection not only saves energy, but has also mechanical advantages, as compared to a rotational motor. In addition, lower inertia, simple structure, speed control and positional maneuverability are obtained. Several designs using linear motors for driving injection have already been disclosed, as further explained in the following.
U.S. Pat. No. 4,895,505 uses a linear motor for driving injection and a coaxial rotation motion for metering, both of which are set on an injection screw and connected by mechanical means.
U.S. Pat. No. 6,051,896 and Japan patent publication no. 2001-300967 both disclose a linear motor for an injection driving device. The motor has a shape like a rectangular tube, in each publication having a particular design of its own, with stators and rotors distributed on the periphery of a rectangle. A metering part is provided, with U.S. Pat. No. 6,051,896 using a rotation motor directly driving an injection screw at a rear end thereof, and Japan patent publication no. 2001-300967 using a rotation motor indirectly driving an injection screw via a belt and a pulley.
Japan patent publications no. 2001-124169 and 2002-079555 both disclose at least two linear motors, connected in series or parallel and linked mechanically for driving injection. As to metering parts, Japan patent publication no. 2001-124169 uses a rotation motor which directly drives an injection screw and is placed at a rear end thereof, whereas Japan patent publication no. 2002-079555 uses a rotation motor indirectly driving an injection screw via a belt and gears.
Above publications all describe one linear motor or at least two linear motors for driving injection; however, for the metering part, a rotation motor is used, performing direct or indirect driving. Rotor and stator of the metering motor move along with the injection movement. Therefore, the injection driving motor not only drives the injection screw and a plastics melting device, but also the metering motor, which increases inertia. If, as partly taught in the patent publications, the stator of the metering motor does not move along with the injection movement, a mechanical transmission system is required, such as a spindle, which causes friction and thus increases load on the motor and reduces mechanical effectiveness thereof. Furthermore, mechanical transmission systems, like belts, pulleys and spindles have backlash, hampering accuracy of positioning and thus impairing precision of manufacturing products. Since the design of the present invention minimizes friction of the assembly and structural parts, good responsive behavior is achieved, and speed response as well as controlling precision are improved.
Moreover, in the driving system of the injection system of the present invention, there are no mechanically transmitting connected parts, so that mechanical impairing of the machine by friction is reduced and the negative effect of backlash between a transmission belt and transmitting parts is avoided, resulting in increased repetitive good behavior.